Alpha-N-acetylgalactosaminide α-2,6-sialyltransferase 1 catalyzes the transfer of sialic acid N-acetylneuraminic acid (Neu5Ac) from cytidine 5'-monophosphono-N-acetylneuraminic acid (CMP-Neu5Ac) to O-linked GalNAc residues in an α-2,6 linkage (1). ST6GalNAcI belongs to a large family of Golgi-membrane-bound sialyltransferases that is responsible for the biosynthesis of sialyated structures, and from which approximately 20 have been cloned to date (2). The cancer associated sialyl-Tn (sTn) angtigen is formed by ST6GalNAcI, through the transfer of Neu5Ac to GalNAc resiudes on mucins (1,3,4). Addition of a sialic acid to a glyco-protein effectively caps the glycan-chain, preventing further additions (5). The presence of sialic acid residues extends the half-life of circulating proteins, and are considered to be important for a large variety of biotherapuetics (6).